1. Technical Field
Embodiments described herein relate to a transmission device and a method of testing a transmission characteristic of a DUT (device under test).
2. Description of Related Art
To evaluate the transmission quality of a data transmission device and a transmission line in terms of a jitter amount, a test signal generator having a jitter generating function and a jitter measuring instrument are used (refer to Non-patent document 1, for example).
FIG. 19 is a block diagram of an example common test signal generator having a jitter generating function. In the following, the same reference symbol that is given to constituent elements shown in different drawings means that they have the same function. In the test signal generator 101, jitter components produced in a noise source 111 by adding together two kinds of periodic jitter PJ1 and PJ2 each originating from sinusoidal noise and random noise originating from Gaussian noise are added to a clock signal or a data signal generated by a clock signal/data signal generation unit 112 and a resulting signal is caused to pass through a DDJ filter 113 for limiting the frequency bandwidth of a transmission system. Thus, a signal having data-dependent jitter (DDJ) is generated.
FIG. 20 is a block diagram showing an example jitter measuring system which uses a test signal generator having a jitter generating function. To measure transmission quality of an evaluation subject transmission line DUT by the jitter measuring system 1000 which uses the test signal generator 101 described above with reference to FIG. 19, it is necessary to jitter-calibrate, in advance, the test signal generator 101 which generates a jitter signal. In jitter calibration, the test signal generator 101 is connected to a jitter measuring instrument 102 by switching of a switch SW. The jitter measuring instrument 102 which is used for jitter calibration is equivalent to a performance monitor 103 for evaluating the transmission quality of the evaluation subject transmission line DUT actually. Individual jitter components of jitter generated by the test signal generator 101 are measured by the jitter measuring instrument 102, and the amount of jitter generated by the test signal generator 101 is adjusted so that the values of the measured jitter components become equal to reference values. To evaluate the transmission quality of the evaluation subject transmission line DUT using the jitter-calibrated test signal generator 101, the test signal generator 101 is connected to the evaluation subject transmission line DUT by switching of the switch SW. A jitter signal generated by the test signal generator 101 is input to the evaluation subject transmission line DUT, and a signal that is output from the evaluation subject transmission line DUT is observed by the performance monitor 103.
FIG. 21 is a sectional view illustrating a probe configuration for observation of an evaluation subject transmission line. The test signal generator 101 and the performance monitor 103 are connected to probe terminals 251 of a probe card 201 and socket terminals 252 of a socket board 202, respectively. The evaluation subject transmission line DUT which is an IC package, an interposer, or the like is placed on the socket board 202 in such a manner that its socket-side terminals 302 come into contact with the respective socket terminals 252, and the probe card 201 pressed against the evaluation subject transmission line DUT in such a manner that probe-side terminals of the evaluation subject transmission line DUT come into contact with the respective probe terminals 251.    [Non-patent document 1] “Controlled Jitter Generation for Jitter Tolerance and Transfer Testing,” Tektronix, Inc., Application Note, 2005 (e.g., see following URL: http://www.tek.com/application-note/controlledjitter-generation-jitter-tolerance-and-jitter-transfer-testing).
The jitter amount of a signal that is output from the evaluation subject transmission line DUT and observed by the performance monitor 103 is influenced by a transmission loss of a signal path called a “test path” from the test signal generator 101 to the evaluation subject transmission line DUT. Therefore, the above-described jitter calibration of the test signal generator 101 needs to be carried out taking the presence of the test path into consideration. To detect a transmission loss of only the test path for the purpose of jitter calibration, it is necessary to realize a “bypass state” that the probe terminals 251 of the probe card 201 are connected to the respective socket terminals 252 of the socket board 202 without intervention of the evaluation subject transmission line DUT.
FIG. 22 is a sectional view illustrating a bypass state that the probe terminals 251 of the probe card 201 are connected to the respective socket terminals 252 of the socket board 202 without intervention of the evaluation subject transmission line DUT. The bypass state could be realized easily if the probe terminals 251 could be connected directly to the socket terminals 252 by pressing the former against the latter. However, where the evaluation subject transmission line DUT is an IC package, an interposer, or the like in which the probe-side terminals are different from the socket-side terminals in pitch and size (see FIG. 21), the probe terminals 251 of the probe card 201 are also made different from the socket terminals 252 of the socket board 202 in pitch and size. Therefore, the probe terminals 251 cannot be connected directly to the socket terminals 252 by simple pressing. In this case, it is necessary to connect (short-circuit) the probe terminals 251 to the socket terminals 252 by transmission lines (hereinafter referred to as “short-circuit transmission lines”) 303, respectively, (see FIG. 22).
However, the short-circuit transmission lines 303 for the connection may have a larger transmission loss than the evaluation subject transmission line DUT. If a transmission characteristic of the short-circuit transmission lines 303 could be measured correctly, they might be used as reference values for jitter calibration. However, measuring a transmission characteristic of the short-circuit transmission lines 303 is more difficult in itself than measuring a transmission characteristic of the evaluation subject transmission line DUT. Furthermore, even if a transmission characteristic of the short-circuit transmission lines 303 could be defined as a jitter calibration reference, it is very difficult to manufacture plural sets of short-circuit transmission lines 303 whose transmission characteristic (absolute values) satisfies that definition, due to the variations in the manufacturing.